In the class of hearing aids generally referred to as implantable hearing aids, some or all of various hearing augmentation componentry is positioned subcutaneously on or within a patient's skull, typically at locations proximate the mastoid process. In this regard, implantable hearing aids may be generally divided into two sub-classes, namely semi-implantable and fully implantable. In a semi-implantable hearing aid, components such as a microphone, signal processor, and transmitter may be externally located to receive, process, and inductively transmit an audio signal to implanted components such as a transducer. In a fully-implantable hearing aid, typically all of the components, e.g. the microphone, signal processor, and transducer, are located subcutaneously. In either arrangement, an implantable transducer is utilized to stimulate a component of the patient's auditory system.
By way of example, one type of implantable transducer includes an electromechanical transducer having a magnetic coil that drives a vibratory actuator. The actuator is positioned to interface with and stimulate the ossicular chain of the patient via physical engagement. (See e.g. U.S. Pat. No. 5,702,342). In this regard, one or more bones of the ossicular chain are made to mechanically vibrate, causing the vibration to stimulate the cochlea through its natural input, the so-called oval window.
In the case of implantable transducers designed to interface with the ossicular chain, precise control of the engagement between the implantable transducer and the ossicular chain is important for proper transducer operation. For instance, stimulation of the ossicular chain, such as through vibration, relies at least in part on the appropriateness of the interface between the ossicular chain and transducer. Overloading or biasing of the implantable transducer relative to the ossicular chain can result in degraded performance of the biological aspect (movement of the ossicular chain) as well as degraded performance of the mechanical aspect (movement of the actuator). Similarly, if the implantable transducer is underloaded relative to the ossicular chain, e.g. a loose connection or no physical contact at all, vibrations may not be effectively communicated.
During implantation, a transducer, such as the one described above, is typically positioned proximate the ossicular chain such that a desired interface or contact with one of the ossicular bones, e.g. the incus, may be made. The transducer position is then fixed using a rigid mounting apparatus, such as a bone anchor, to maintain the position of the transducer and thereby the desired contact with the ossicular chain. As will be appreciated, however, such a system maintains the position of the implanted transducer relative to the ossicular chain, but does not maintain the position of the ossicular chain relative to the implanted transducer, such that an ossicular movement (other than those intentionally caused by the transducer) due to a physiological change may affect the interface between the ossicular chain and implanted transducer. In other words, ossicular movement due to a physiological change, referred to as a “physiological movement,” may naturally occur because of a variety of circumstances including: changes in barometric pressure (e.g. caused by changes in altitude of the patient), tissue growth, swallowing, swelling after transducer implantation, and/or even clearing of the ears. Since the transducer is rigidly mounted, physiological movements of the ossicular chain may affect the interface with the transducer, e.g. resulting in an under or over loaded engagement with the transducer. This in turn may be realized in the patient by a “drop-off” in hearing function.
During normal operation of an implanted transducer, it is desirable to focus acoustic stimulation energy toward an auditory component (e.g. a component of a patient's biological hearing system) to be stimulated. It is also desirable to isolate the stimulation energy to minimize resonant phenomena due to re-amplification of feedback signals over a feedback path leading to the microphone. For instance, in the case of an implantable transducer mounted to a patient's skull as described above, vibrations from the transducer may be transmitted via the mounting system to the patient's skull and thereafter to the microphone when the transducer gain reaches a certain level. This in turn may limit the maximum gain available in a transducer, e.g. the higher the gain the higher the likelihood of resonant phenomena due to re-amplification of feedback signals. It is therefore desirable that the intensity of the vibration transmitted to the skull from an implantable transducer be reduced, making it possible to transmit a correspondingly larger intensity of vibration to a patient's middle ear without feedback. This in turn results in a higher maximum available gain in the transducer, and more efficient transducer operation.